MB-7735: Fix to a race in notifying / releasing TAP connections

Releasing and notifying an TAP conneciton is now synchronized
through a separate lock.

Change-Id: I36deeeaab98235500d9ba22c213c413eac1fbc04
Reviewed-on: http://review.couchbase.org/26705
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>